A conventional cushion device for treadmills is disclosed in U.S. Pat. No. 5,827,155 and includes a leaf spring connected between the side rails of the frame. The free ends of the leaf spring are connected to downward feet. The is a proper height between the leaf spring and the frame so that when the users are jogging one the endless belt of the treadmill, the deck for supporting the users is flexible and of the treadmill. An adjustable device includes a lever and a rack which is engaged with an adjustable toothed member so that when operating the lever, the distance between the rear part of the leaf spring and the frame can be adjusted so that the spring or cushion force can be adjusted.
However, the rear part of the frame is supported by one piece of leaf spring so that the frame shakes at a frequency which might not be the same as the frequency as the users jogging on the endless belt. In other words, the moving deck together with the endless belt confuses the user where the exact position of the endless belt is and this might cause ankle injury of the users. Besides, the length of the leaf spring has to be limited. If the leaf spring is too long and in contact with the frame, there will be no flexibility. If the leaf spring is too short, the flexibility is limited in a narrow range which is not satisfied by the users.
The present invention intends to provide a cushion device for treadmills wherein the deck is supported by two sets of cushion assemblies which absorb the shakes and vibration.